The present invention relates to so-called gamma-gamma logging techniques for determining characteristics of earth formations surrounding boreholes by irradiation of the formations with gamma rays and detection of gamma rays that are scattered back to the borehole. More specifically, the invention concerns gamma-gamma logging methods and apparatus for determining the thickness of the media between a logging sonde and the surrounding earth formations.
As is well known in the well logging art, the density of earth formations surrounding a borehole can be measured by emitting gamma rays in the borehole from a collimated source contained within a logging sonde and detecting gamma rays that are scattered back to the borehole. As the gamma rays from the source interact principally with free or loosely bound electrons in the scattering medium, the amount of detected gamma radiation is related to the density of the medium, which can thus be determined.
One of the major problems encountered in gamma-gamma logging arises from the fact that the sonde is almost always separated from the adjacent formation by the mudcake present on the borehole wall or by a casing and the associated cement annulus. As the materials between the sonde and the formations are traversed twice by the gamma rays, the formation density measurements are strongly affected by the densities of these materials, which are generally different from the formation density.
To obtain more accurate formation density readings, it has been proposed to use two radiation detectors spaced at different distances from the gamma ray source. The near or short-spaced detector receives gamma rays which have diffused principally in the materials near the borehole wall. The far or long-spaced detector receives gamma rays which have diffused principally in the formation. Moreover, in order to eliminate the influence of the average atomic number of the elements in the scattering medium, the count rate of the detectors is measured in a restricted energy range covering only gamma rays which have been degraded by Compton effect. A value of uncompensated or apparent formation density is derived from the count rate of the far detector, and a density correction is computed using the two count rates. The density correction is then applied to the apparent density to obtain the true formation density. The technique is described in U.S. Pat. No. 3,321,625 issued on May 23, 1967 to J. Wahl and assigned to the assignee of the present invention.
It is easily conceivable that the above compensation technique provides an accurate correction of the formation density readings only as long as the near detector can "see" through the materials between the sonde and the formation. It would certainly be possible to make the near detector sensitive to gamma rays which have penetrated more deeply into the adjacent media by increasing the spacing between the source and the detector. However, the source-detector spacing must be kept reasonably low to permit statistically accurate measurements while the sonde is being moved through the borehole at a reasonable speed, without having to dangerously increase the strength of the source. For this reason, it is generally impossible to obtain a satisfactory correction, in uncased boreholes, for mudcake thicknesses exceeding 1.5 inches and, in cased boreholes, for cement thicknesses exceeding 1 inch. The mudcake thickness is generally lower than 1.5 inches, but the cement thickness often exceeds 1 inch. Therefore, it is desirable to obtain an indication of the thickness of the layer of cement or any other bonding material between the sonde and the formation for use as an index of the reliability of the corrected density readings.
Accordingly, it is a broad object of the present invention to provide gamma-gamma logging methods and apparatus for determining the thickness of the media between a logging sonde and the adjacent formations.